Hydraulic actuation systems are known for providing mechanical control of movable elements within various systems or vehicles including aircraft and ships. For example, a hydraulic actuation system may be used to adjust the pitch of the propeller blades of a variable-pitch propeller.
A hydraulic actuation system generally comprises a hydraulic actuator comprising a cylinder and a piston that slides within the cylinder, and a valve (such as an electrohydraulic servo valve (EHSV)) for controlling how hydraulic fluid is ported to the cylinder, and hence controlling the position of the piston within the cylinder. The combination of the hydraulic actuator and a servovalve may be referred to as a “servoactuator”. The hydraulic actuator may generally be single acting, such that hydraulic pressure is applied to just one side of the piston, or double acting, such that hydraulic pressure is applied on each side of the piston whereby the pressure difference between each side causes the piston to move.